A US government report shows that pregnancy rates for women in their early 20s dropped nearly 18 per cent between year 2000 and 2008 while rates for women in their 40s have jumped 65 per cent. In fact, rates for all age groups over 30 have increased thanks, apparently, to women investing more in careers.
But that delay is not universal. The pregnancy rate for women in their late 20s has remained stable, and even though the rate for the early 20s has dropped slightly behind the older group it is still high compared to the rest. In short, women in their 20s are still having most of the babies.
What they are not doing is getting married. More than half of births in this age group occur outside marriage. The New York Times Motherlode blog observes:

The more educated a woman is, the more likely she is to have children later in life, and the more likely she is to be married before she has a child. This small set of demographics doesn’t just reflect changes in the age at which women become pregnant. It’s a tiny snapshot of a growing socioeconomic divide among parents: one that has an impact on our priorities and our politics as well as our culture as a whole. What changes in a country where a growing number of women are becoming mothers as others in their cohort become grandmothers? We’re going to find out. There is quite a lot in the official report about teenagers and pregnancy, which I’ll look at another day. Meanwhile it is noteworthy that abortion rates have fallen for both the married and unmarried. However, counting abortions is an inexact science in the US and the Guttmacher Institute, which is notorious for questionable abortion estimates, has had a hand in this report. Just keep that in mind.